1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machine or printer, which is loaded with a multi-layered electrophotographic photoreceptor having an undercoat layer.
2. Description of Related Art
With the progress of the development, conventionally used inorganic materials typified by amorphous selenium and amorphous silicone have recently been replaced by organic photoconductive materials. An electrophotographic photoreceptor using an organic photoconductive material is slightly inferior in sensitivity, durability and stability in the environment, but has a lot of merits in toxicity, cost and degree of freedom of material design as compared with the inorganic material.
In the electrophotographic photoreceptor, there is widely used an organic photoreceptor comprising a charge generating material which generates charges upon irradiation with light, a charge transporting material which transports charges generated, and a binder resin constituting a layer in which these substances are dispersed. Commonly, the organic photoreceptor is roughly classified into a photoreceptor comprising a single-layered photosensitive layer wherein a charge generating material and a charge transporting material are contained in the same layer, and a photoreceptor comprising a multi-layered photosensitive layer formed by laminating a charge generating layer containing a charge generating material with a charge transporting layer containing a charge transporting material.
A multi-layered photoreceptor obtained by directly forming a multi-layered photosensitive layer on a conductive substrate through coating is likely to be influenced by the surface of a conductive substrate and it is difficult to form a layer uniformly and homogeneously and therefore thickness unevenness occurs, thus causing various image defects and density unevenness. Also, since a layer containing a charge generating substance is directly contacted with a conductive substrate, when electric field is applied by charging, the charge generating substance partially generates charges, and thus the potential locally decreases at the position where the charge generating substance exists in the vicinity, and problems such as blank paper and fog at the gray portion occur in the reversal development. These problems conspicuously occur under high temperature and high humidity environment.
It is known to be effective to provide a resin layer, which is referred to as an undercoat layer or an intermediate layer, between a conductive substrate and a photosensitive layer in case of a single-layered photoreceptor and to provide the resin layer between a conductive substrate and a charge generating layer in case of a multi-layered photoreceptor so as to solve these problems. For example, a layer formed by coating an alcohol soluble polyamide resin and drying the resin is considered to be effective as the undercoat layer (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 52-25638 and Japanese Examined Patent Publication (Kokoku) No. 63-018185).
Even if such an undercoat layer is provided, although good electrical characteristics and image quality are obtained at the initial stage, the alcohol soluble resin shows a large change in resistance by the environment such as temperature or humidity and thus a potential conspicuously changes with the environmental change, resulting in defects such as black spots, memory and density unevenness on images.
Thus, a photoreceptor having an undercoat layer made of titanium oxide is proposed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 56-52757). Furthermore, there is also known a technology about a surface treated titanium oxide for the purpose of improving image characteristics or improving dispersibility of titanium oxide in a coating solution for an undercoat layer.
Specifically, Japanese Unexamined Patent Publication (Kokai) No. 2-181158 proposes titanium oxide coated with alumina, and Japanese Unexamined Patent Publication (Kokai) No. 9-152731 proposes coating of titanium oxide with alumina and silica. Also, Japanese Unexamined Patent Publication (Kokai) No. 9-258469, Japanese Unexamined Patent Publication (Kokai) No. 4-229872 and Japanese Unexamined Patent Publication (Kokai) No. 8-328283 propose coating of titanium oxide with a reactive organosilicon compound. Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2002-236381 proposes coating of titanium oxide with alumina, silica and siloxane.
However, the particle size of titanium oxide has never been optimized with respect to electrical characteristics and image characteristics of a photoreceptor, and particles having a number average primary particle size of 20 to 100 nm were mainly used as ultrafine titanium oxide particles and particles having a number average primary particle size of 0.1 to 1.0 μm were used as a pigment grade titanium oxide.
With the size reduction of a copying machine and a printer, LED has recently been used as exposure light in the formation of a latent image. However, there has been known a specific peculiar image problem such as interference fringe (moire) caused by reflection on the surface of a conductive substrate in the formation of a latent image by LED or laser. Therefore, in the prior art, an undercoat layer containing titanium oxide having average primary particle size of 1 to 20 μm was used so as to prevent the occurrence of the interference fringe.
However, when an organic photoreceptor having an undercoat layer containing titanium oxide having average primary particle size of 1 to 20 μm is used, there arose problems that sensitivity deteriorates during continuous use and fog occurs under high-temperature and high-humidity severe environment.